The invention is directed toward subsurface surveying such as well logging. More particularly, the invention is directed to improved neutron and photon detectors for subsurface surveying applications. The term "photon" as used herein includes both gamma rays and X-rays.
Nuclear techniques have been extensively applied to oil exploration, particularly in the area of well logging. Nuclear particle detectors are used to find strata containing oil and natural gas. These investigations can estimate the extent of fuel-bearing strata and the amount of fuel that these strata contain.
The scientific basis for this survey technique is based on the fact that different materials undergo different reactions when irradiated with neutrons. After a stratum is irradiated with neutrons, photons and/or neutrons returning from the stratum are detected to obtain information regarding the stratum.
In this technique, a probe, or sonde, containing a neutron source is drawn through a borehole and a detector in the sonde measures the energy and/or intensity of radiation returning from the strata. Oil, gas, water, and various other geological formations possess a distinctive radiation signature that permits identification of the make-up of the strata.
Borehole detectors of high-energy photons such as gamma rays and X-rays returning from the strata employ specially grown scintillation crystals which produce a flash of visible or near-visible photons when a high-energy photon interacts with atoms in the crystal. This flash of visible or near-visible photons is sensed in a photomultiplier tube (PMT) which is adjacent to the scintillation crystal and the PMT produces an electrical signal indicative of the flash intensity. The flash intensity is dependent upon the energy of the incident high-energy photon.
These electrical signals are transmitted to the surface where they are analyzed. Analysis of the time and energy spectrum distributions of the detected high-energy photons provides information about subsurface conditions.
Two types of neutron sources are generally used in the downhole probe. One type employs radioactive sources, such as americium/beryllium or californium, that continuously emit neutrons. The other type of neutron source employs a pulsed 14 MeV neutron generator.
Each of these types of neutron sources has its disadvantages. Radioactive sources require special licenses in every country where the source will be used. Radioactive sources require bulky shielding and complicated transportation arrangements and are occasionally lost downhole, which can render the hole unusable due to radiological concerns. Radioactive sources do, however, possess the significant advantage of having a well-known neutron emission rate.
Pulsed generators are safer to use and permit a wider variety of downhole measurements. However, pulsed generators do not emit neutrons at a constant rate. Therefore, a neutron monitor must be used downhole to monitor a pulsed neutron generator. The neutron monitor measures the number of neutrons emitted by the neutron generator, not the number of neutrons returning from the strata. Information concerning the number of neutrons emitted by the neutron generator is used to compensate calculations for variation in the neutron output of the neutron generator.
Further background on conventional subsurface surveying and surveying equipment is provided in U.S. Pat. No. 5,008,067, issued to John B. Czirr on Apr. 16, 1991. The entire contents of the '067 patent are incorporated herein by reference.
The neutron monitoring technique disclosed in the 067' patent is based on the .sup.16 O (n,p) .sup.16 N reaction. In this reaction, a neutron n from a neutron generator enters and reacts with an oxygen containing scintillation material according to the following reaction: EQU n+.sup.16 O.fwdarw..sup.16 N+p Equation (1)
The .sup.16 N decays rapidly due to its seven second half-life. The decay of the .sup.16 N results in the production of high-energy gamma rays and electrons. The high-energy gamma rays and electrons cause scintillation material in the neutron monitor to scintillate, that is, to produce a flash of light. This light is detected, converted into an electrical signal, and amplified to provide an indication of the neutron output of the neutron generator.
Unfortunately, the scintillating materials disclosed in the '067 patent, bismuth germanate (BGO) and lithium glass, have numerous disadvantages. Bismuth germanate does not scintillate at the high temperatures frequently encountered downhole. Lithium glass has a low atomic number and is thus not well-suited to detect the gamma rays and electrons produced during .sup.16 N decay.
One possible material choice for a neutron monitor is cerium-activated gadolinium silicate. Unfortunately, the high thermal neutron capture probability of cerium-activated gadolinium silicate causes the gadolinium silicate to be unacceptably influenced by background thermal neutrons.
Other scintillating materials that have been considered for downhole use do not effectively scintillate at the high temperatures frequently encountered downhole.
Thus, there is a real need for neutron detection materials which can efficiently operate at the temperatures encountered in the course of subsurface surveying and which can effectively detect high-energy gammas and electrons resulting from .sup.16 N decay.
Another problem in the design of subsurface surveying equipment relates to the choice of a scintillation crystal to detect photons coming from the strata. The borehole environment is hostile to electrical and mechanical equipment in that the borehole environment is wet, contains corrosive materials, and experiences high pressures and temperatures. Furthermore, the space in a borehole is very limited. In addition, because the maintenance and operation of a borehole drilling rig is extremely expensive, the time spent on subsurface surveying has to be minimized. These conditions and requirements impose severe constraints on the materials and construction of a subsurface surveying sonde.
A scintillation crystal to detect photons coming from the strata should ideally have the following properties:
(1) a high density to maximize the number of interactions between high-energy photons coming from the strata and the scintillation crystal; PA1 (2) fast decay of the scintillation process following a gamma ray or X-ray interaction and low afterglow; PA1 (3) high scintillation light output linearly related to incoming photon energy to provide adequate energy resolution and measurement; PA1 (4) high transparency to minimize attenuation of the light flashes within the scintillation crystal; PA1 (6) insensitivity of the scintillation process to temperature changes; and PA1 (7) mechanical strength and resistance to the corrosive effects of the materials encountered in the borehole environment.
Many scintillator materials fail to provide an acceptable combination of these properties. Thus, there is a need for improved scintillation crystals to detect photons coming from the strata.